As the dimensions of lithographically defined features become smaller, effects that were not significant at larger dimensions become important. For example, feature sizes may have identical size variations, and the variations may be acceptable for large features but unacceptable for small features because the variation is too high a percentage of the small feature size. Thus, critical dimension control is important for small feature size. Variations in dimensions may, of course, arise from many sources.
Integrated circuit features are typically defined with a lithographic pattern delineation process. A photosensitive material, commonly termed a resist, is formed on the substrate surface and portions of the resist are selectively exposed to radiation which creates a differential removal rate between the exposed and unexposed portions. The portions of the resist with the more rapid removal rate are now removed by, e.g., etching, to expose selected portions of the underlying substrate surface. Further processing, such as etching of the exposed substrate or ion implantation, etc., is now performed.
Ideally, all of the incident radiation travels a vertical path through the resist and is completely absorbed before reaching the bottom of the resist. This is, however, not always true in practice. Radiation may travel entirely through the resist and be reflected back into the resist. The reflection may be non-specular or from a non-horizontal surface, and the reflected radiation may be absorbed in portions of the resist that are not desirably exposed to radiation. The reflected radiation, of course, degrades the quality of the pattern delineation process because it is absorbed where it should not be absorbed.
To reduce the amount of reflected radiation, materials termed anti-reflection coatings have been developed. These materials are positioned underneath the resist. They are frequently organic polymers such as polyimides. Of course, polyimides are used for other purposes, such as interlevel dielectrics, in integrated circuit fabrication. For use as an interlevel dielectric, the polyimide frequently has to be patterned to form vias which expose portions of the underlying material. Of course, portions of the anti-reflection coating must also be patterned to expose the underlying substrate material. The use of polyimides in integrated circuits is discussed by Lai et al. in Industrial Engineering Chemical Product Research Development, 25, 1986, pp. 38-40. Consideration of the above shows that there must be an etching process for the polyimide.
Plasma etching of polyimides is discussed by Scott et al. in Journal of Vacuum Science and Technology, A8, May/June 1990, pp. 2382-2387. Scott used a mixture of CF.sub.4 and O.sub.2 and found that the addition of CF.sub.4 increased the etch rate of the polyimide as compared to the rate obtained with only O.sub.2. The increased etch rate was attributed to both etching by the CF.sub.4 and the formation of atomic O in the plasma by the CF.sub.4. Turban et al., Journal of the Electrochemical Society, 130, November 1983, pp. 2231-2236, also reported the etching of polyimide using O.sub.2 and either CF.sub.4 or SF.sub.6. Tepermeister et al., Journal of Vacuum Science and Technology, A9, May/June 1991, pp. 790-795, reported the etching of polyimides using Ar, O.sub.2 and O.sub.2 /F.sub.2 plasmas. They reported that the etching process was a combination of gas phase plasma chemistry and plasma surface interactions. See also, Kogoma et al., Plasma Chemistry and Plasma Processing, 6, 1986, pp. 349-380, for a discussion of etching mechanisms of polyimide in O.sub.2 /SF.sub.6 plasmas.
Of course, methods for detecting the endpoint of plasma etching processes are also known. See, for example, U.S. Pat. No. 4,312,732, issued on Jan. 26, 1987 to Degenkolb et al.
Consideration of the results of the above processes indicates that there is a need for a method of etching polyimides which provides critical dimension control and provides for an optical endpoint capability.